Better :in_app_module_allow_list default

[sl0thentr0py]

Currently in_app_module_allow_list defaults to an empty list, so most users don't get good in-app frames marked.
Ruby for instance defaults to a combination of project root + known common directory names
https://github.com/getsentry/sentry-ruby/blob/c3bcfa0401856d9ee7616c95ab9dfe988a89da30/sentry-ruby/lib/sentry/backtrace.rb#L91-L93

[whatyouhide]

@savhappy we could use :application.get_key(app, :modules) to get all the app modules.

Basically, we could do this if :in_app_module_allow_list is not set:

1. Check if we're in an umbrella project (Mix.Project.umbrella?/1).
2. If yes, then we get all the apps from the umbrella and do what's described below, otherwise just do it for the root app.
3. Get the app name for the current Mix project.
4. Get the modules with :application.get_key(app, :modules).

This might not be trivial work, want me to give it a try first?

[savhappy]

@whatyouhide yep! Go for it! If you need me to take some of the work load let me know

[sl0thentr0py]

yep sounds good to me, thx both!